About 7-8 million child deaths occurred in 2010, mostly in South Asia and sub-Saharan Africa. Strikingly, underutilization of basic child health services is highly prevalent in a large number of low- and middle-income countries (LMICs), and inequality in child mortality is large. Measles has been a key contributor to child mortality, and can lead to significant sequelae and long-lasting effects on child development. The World Health Organization strategy to reduce global measles mortality includes maintaining high coverage for routine immunization of the first dose of measles vaccine (MCV1) and ensuring all children receive a second vaccine dose (MCV2). In the majority of LMICs, MCV2 is offered through supplemental immunization activities (SIAs). SIAs are mass campaigns which occur every 2-4 years during which health workers establish outreach service points or go door-to-door to offer immunizations to all children of a given age range. Experience from the Americas and sub-Saharan Africa has shown that SIAs can achieve high levels of measles control. In addition, measles SIAs often incorporate the delivery of other child interventions including oral polio vaccines, vitamin A supplements, deworming medicines, and insecticide-treated bed nets. Due to the large number of children reached, SIAs mobilize considerable health workforce and financial resources. Although questions about the effects of mass campaigns on routine services have been raised, analyses on how SIAs may strengthen/weaken health systems remain scarce. SIAs may induce positive synergies within health systems, in terms of planning, financial resources, workforce, delivery, surveillance systems. As routine activities rarely achieve universal coverage in a population, SIAs may reduce coverage heterogeneity and may achieve equitable access to basic child health services. On the other hand, SIAs may have negative effects in countries which face a shortage of health workers and inequitable distribution of the workforce, by perhaps diverting the workforce from accomplishing routine duties when directed to work on the SIAs. This detrimental impact on basic health services may lead to increases in mortality. We will systematically compile a large global database, primarily assembled from large nationally- representative household-based surveys. Using quantitative methods including interrupted-time series (ITS) designs, we will examine the positive/negative interactions of SIAs with selected health services, to identify how SIAs strengthen/weaken health system functioning; we will estimate the differential impact of SIAs on different socio-economic groups and determine their equity effects. In this proposal, we will assess the broader benefits/disadvantages of SIAs so that policymakers better understand the sensitive trade-offs between distinct health delivery platforms, here SIAs vs. routine immunization services. Our study will enable better resource allocation so that an increasing number of children receive life-saving interventions in order to save the largest number of lives at a lower cost.